For example, in systems for performing transmission of moving picture (image) signals to remote place like television conference system and/or television telephone system, when moving picture signals are digitized as they are, information quantity extremely large. However, image signals are compression-encoded by making use of line correlation or interframe correlation of video signals to thereby have ability to decrease information quantity. As a result, transmission of many moving pictures (images) can be made at the same time, and moving pictures (images) of long line can be recorded with respect to a recording medium in its turn. As the representative system of efficient encoding system for moving picture (image), there is MPEG system. In the MPEG system, code quantity control is performed so that bit stream sent to a transmission path has a desired rate.
FIG. 1 shows the configuration of a conventional image compression encoding apparatus 7 represented by the MPEG system. This conventional image compression encoding apparatus 7 realizes irreversible image compression on the basis of the DCT encoding system of performing DCT of inputted image signal thereafter to quantize the image signal thus transformed. The image compression encoding apparatus 7 comprises a terminal 71, a block dividing unit 72, a DCT unit 73, a quantization unit 74, a variable length encoding unit 75, and a buffer 79.
In this FIG. 1, the terminal 71 is supplied with an image signal consisting of luminance signal Y, and color difference signals Pb, Pr.
The block dividing unit 72 serves to divide inputted image signal of one frame into, e.g., 8×8 blocks to output them to the DCT unit 73.
The DCT unit 73 implements DCT every 8×8 blocks to thereby generate DCT coefficients to output the DCT coefficients thus generated to the quantization unit 74. The quantization unit 74 quantizes DCT coefficients inputted from the DCT unit 73 by quantization step determined on block basis. The quantization unit 74 delivers the quantized data (hereinafter referred to as quantization level) to the variable length encoding unit 75. The variable length encoding unit 75 performs variable length encoding of the quantization level delivered from the quantization unit 74 by method, e.g., two-dimensional Huffman encoding or arithemetic encoding, etc.
In this respect, code quantity control at the image compression encoding apparatus 7 is performed by controlling quantization step. There are also instances where such code quantity control may be performed by conducting a feedback control by using, e.g., the remaining quantity of virtual buffer and the relationship between quantization step when encoding has been conducted in the past and generated code quantity.
It is to be noted that, in the image compression encoding apparatus 7, it is required to perform code quantity control so that total code quantity generated at one frame (equi-length unit) is not above set target code quantity at all times. However, when large code quantity is left as total code quantity in order to satisfy target code quantity, image quality would be rather deteriorated. For this reason, in the prior art, there is proposed a code quantity control method for using up total code quantity while satisfying target code quantity. In this code quantity control method, code quantity generated in equi-length unit is calculated in advance at plural quantization steps to determine suitable quantization step within the range where generated code quantity is not above target code quantity, and is proposed, as feed-forward system, in, e.g., International Laid Open Publication WO 96/28937.
In the conventional code quantity control method of the feed-forward system, etc., when the quantization step is enlarged, quantization becomes coarse so that total code quantity becomes small. On the other hand, when the quantization step is reduced, quantization becomes fine so that total code quantity also becomes large. Moreover, since only discrete values can be acquired at quantization, the total code quantity also becomes discrete. Namely, the quantization step is controlled on block basis, thereby making it possible to control total code quantity. In order to use up total code quantity while satisfying the target code quantity, it is necessary to select, every block, quantization step which is less than target code quantity and which is minimum.
However, in the above-described code quantity control method based on the DCT encoding system, when DCT coefficients are quantized by quantization step selected, e.g., on block basis, any distortion may be added in the process of compression and expansion of image, it is impossible to completely maintain original image quality. For this reason, it was necessary to employ the reversible encoding (Lossless) system which can preserve original information via processes of compression and expansion like, e.g., ideally DPCM while realizing such a code quantity control which is not above set target code quantity to thereby prevent deterioration of picture quality.